UAVs are widely used for military and non-military uses. Roles for UAVs include reconnaissance and offensive strike missions. Adoption of UAVs for use aboard ships, however, is limited largely because of the challenges of recovery at sea. The challenges include small flight decks and wave-induced ship motion.
Methods of recovery that have been employed at sea include deck-mounted nets and water landings. Drawbacks to net-based capture include the risk of damage to the UAV and the potential for the UAV to be ensnared in the net. Drawbacks to water landing include the necessity to modify the UAV heavily for water landings and the need to recover the UAV from the water after landing.
Other systems for shipboard UAV recovery, based instead on arresting lines, are disclosed in U.S. Pat. Nos. 7,059,564 and 7,219,856. The disclosed system in U.S. Pat. No. 7,059,564 includes a cable hanging vertically from a boom extending out over the side of the ship. The UAV with special fastener devices at the wing tips is flown into the hanging cable and then captured when the cable slides into one of such devices and becomes attached to the cable. The disclosed system in U.S. Pat. No. 7,219,856 includes a boom that holds a line over the side and parallel to the deck of the ship. The UAV with an attached hook that is disclosed in U.S. Pat. No. 7,143,976 is flown above the line and then captured when the hook snags the line. These recovery systems offer distinct advantages over net-based and water landing methods. An important advantage over the net-based method offered by both of these systems is that recovery occurs over the side of the ship, which reduces the risk of collision with the ship and allows recovery to occur outside the area of most intense turbulence caused by the air wake of the ship superstructure. A drawback of the system disclosed in U.S. Pat. No. 7,059,564 is that the UAV must be designed to accommodate a severe turning moment caused by the arresting line force exerted on a wing tip. In addition, the airframe of the UAV must be heavily modified so that the leading edges of the airframe and wings can withstand impact with the hanging cable and that the cable can slide reliably to a wing-tip fastener. This system has been successfully operated using small UAVs (i.e., under 50 lbs. weight), but is unlikely to be scalable to UAVs of middle or large size (e.g. 200-1000 pounds), due to the higher energies involved in turning moments and cable impacts when masses are greater, yet fixed materials strength. A drawback of the system disclosed in U.S. Pat. No. 7,219,856 is that it cannot accommodate significant vertical flight path errors that are caused by wind buffeting or guidance errors. In addition, the boom will rotate upwards and downwards as the ship rolls and heaves, seriously complicating the hook capture task. A further drawback of both of these recovery systems is that, after arrest, the UAV is left dangling in a near-vertical orientation, thus complicating handling and placement on deck.
As a consequence, there is a need for a UAV recovery system that, in addition to the capability of capturing a UAV over the side of a ship, can accommodate a wide range of UAV sizes, large, wave-induced ship motions, and substantial vertical flight path errors of the incoming UAV, and can easily handle the UAV after capture.